Hypnos, the personification of sleep, is one of the children of Nyx, Night, and the twin of Thanatos, Death, a pairing the Greeks felt keenly, sleep being death's gentle likeness, and the two are shown together in art carrying the body of the slain, most famously the dead Sarpedon on a celebrated Athenian vase. He is more a figure of poetry than of cult, with few if any temples, but his mythic role is precise: in the Iliad Hera bribes him to lull Zeus so the battle may turn, a dangerous errand he undertakes only because he has been burned once before and fears the god's anger. He is father of the dream-spirits, the Oneiroi, chief among them Morpheus. His Roman name is Somnus, and through Ovid and later poetry his dark cave, past which the river of forgetfulness flows, became a stock image of drugged repose.
A later strand of myth, recorded by Roman writers, makes Hypnos, rather than Selene alone, the agent of Endymion's endless sleep, granting the beautiful shepherd eternal youth precisely by keeping him forever unconscious, a variant that folds the moon-goddess's love story into the sleep-god's own domain. Ovid's Metamorphoses locates Somnus's cave in the land of the Cimmerians, where no dawn ever reaches and a stream murmuring of the river Lethe induces drowsiness in anyone who hears it, an image of drugged, sunless repose that shaped how later European poets pictured the god's realm long after his cult, such as it was, had vanished.